1. Field of the Invention
The present invention generally relates to hydrostatic transmissions and transaxles and, more particularly, is concerned with a fan and pulley arrangement for providing external cooling for a hydrostatic transmission or transaxle of either the radial piston or swash-plate type.
2. Description of the Prior Art
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, then back to rotary mechanical motion to rotate a pair of drive axles in order to drive a vehicle such as a lawn and garden mower or tractor. The hydrostatic transmission regulates or controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. This type of hydrostatic transmission is shown in U.S. Pat. No. 4,979,583 entitled Variable Speed Transaxle, issued Dec. 25, 1990, and pending application Ser. No. 07/535,462 entitled Variable Speed Transaxle filed Jun. 8, 1990 both of which are specifically incorporated by reference herein.
Because of the circulating oil within the hydrostatic unit, it is necessary to provide external cooling to maintain the oil at a proper temperature and to ensure that the unit does not become overheated which can degrade the oil or the performance of the hydrostatic transmission. External cooling is typically done by circulating air over the transmission casing in order to dissipate the heat conducted from the oil to the casing into the moving air.
Prior art solutions to external cooling have typically included providing a fan attached to the input shaft which rotates with the shaft thereby circulating air over the transmission casing. Some prior art solutions have also included, in addition to the provision of a fan, straight ribs formed on the transmission casing to increase the surface area in order to dissipate more heat as the fan blows air over the ribs. These prior art ribs, instead of providing more efficient cooling, actually act as impediments to proper and efficient air flow over the transmission casing. This may cause an excess buildup of heat since the air flow generated by the fan is generally helical while the ribs formed on the transmission casing are straight, thus the helical air flow generated by the fan must undergo a change in direction as it deflects off of the ribs causing air turbulence over the transmission casing and between the fan and ribs, thereby recirculating the heated air rather than providing fresh, cool air. Therefore, continuous cooler air is not circulated over the transmission casing which would provide faster and more efficient cooling.
Thus, these prior art solutions often do not provide adequate cooling of the transmission and therefore the hydrostatic transmissions may suffer performance and life expectancy degradation due to undissipated heat buildup within the transmission because of the inadequate external cooling.
The present invention is thus directed towards eliminating the aforementioned problems associated with external cooling of hydrostatic transmissions.